This application is based on application No. 10-2000-0000892 and No. 10-2000-0020101 filed in the Korean Industrial Property Office on Jan. 10, 2000 and Apr. 17, 2000 respectively, the contents of which are incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a high crystalline polypropylene microporous membrane and a preparation method of the same, and more particularly to a high crystalline polypropylene microporous membrane which is prepared through the steps comprising preparing a precursor film using high crystalline polypropylene having a 50% or more crystallinity and a very high isotacticity, annealing, stretching at a low temperature, stretching at a high temperature, and heat setting, and a preparation method of the same.
Furthermore, the present invention relates to a multi-component micro-porous membrane and a preparation method of the same, and more particularly to a microporous membrane comprising two or more synthetic resin components, different from a conventional microporous membrane comprising one component of polypropylene, and a preparation method of the same.
(b) Description of the Related Art
A microporous membrane is being widely used in various fields such as filter membranes in air purification, water treatment, etc., separators in electrolysis, batteries, etc., gas exchange membranes, artificial internal organs, beverage purification, enzyme refining, etc. The importance of a microporous membrane as a separator in batteries, particularly in lithium ion batteries, is increasing.
A separator for batteries, particularly a separator for lithium ion batteries among the applications, is described in the following.
As separators for batteries, these membranes play a role that isolates an anode from a cathode, and thereby prevent short circuits due to melting contacts between two electric poles, and at the same time passing electrolyte or ions. Although varieties of separators are being used depending on the type of batteries, chemical materials of electrolytes, etc., various studies on separators for lithium ion batteries are recently being pursued since separators for lithium ion batteries require different characteristics than separators used in conventional batteries. Although there are cellulose, nonwoven fabric, etc. as general separator materials used in conventional batteries, a novel microporous separator using synthetic resin materials has been developed since it is difficult for conventional separator materials to satisfy aforementioned characteristics required in batteries.
There are many cases in which polyolefin based resin having a low reactivity with an organic solvent and a low manufacturing cost is used as a material for separators because highly active organic solvents are used as electrolyte in lithium ion batteries. However, there are no materials apart from polyolefin resin which have practically been used up to now as separators for lithium ion batteries.
Although methods for preparing a precursor film by using polyolefin based resin include many processes which can be used theoretically or in laboratories, methods for preparing a microporous membrane used as a separator which is now commercially available can largely be divided into a wet process, in which filler or wax, and solvent are used, and a dry process in which solvents are not used. Furthermore, the stretching process related with forming of microscopic pores from these methods frequently employs mono-axial stretching and double-axial stretching methods.
A dry process can prepare a wider width of precursor film above all, has a relatively easier production process than a wet process, and does not use solvents, thereby having merits of a superior manufacturing environment and easier mass production.
A preparation method of a microporous membrane using a dry process includes the continuous cold stretching and hot stretching processes as disclosed in U.S. Pat. Nos. 3,679,538 and 3,801,692, etc. Generally, these processes include a series of processes in which membrane formation is completed by heat setting after forming micropores on the film by passing a precursor film having a high crystallinity and elasticity through a cold stretching process and then continuously stretching the film at a high temperature. Particular technologies on the continuous cold stretching and high temperature stretching processes are disclosed in U.S. Pat. Nos. 3,843,761 and 4,238,459. The disclosed technologies include methods in which the annealed precursor film is initially cold stretched and then multi-step high temperature stretched. Furthermore, it is disclosed in U.S. Pat. No. 5,013,439 that a membrane of which pore size is decreased and pore density is increased can be prepared using a multi-step cold stretching process in the continuous cold stretching and high temperature stretching processes. It is disclosed in U.S. Pat. Nos. 5,385,777 and 5,480,745 that a microporous membrane is prepared through the aforementioned continuous multi-step cold stretching and high temperature stretching processes using a blend of polyethylene and polypropylene in order to improve safety of lithium ion batteries.
Furthermore, a preparation method of a separator for lithium ion batteries by laminating polyethylene and polypropylene using a dry process is disclosed in European Patent Publication Nos. 715,364, 718,901 and 723,304, U.S. Pat. Nos. 5,240,655, 5,342,695 and 5,472,792, Japanese Patent Laid-open Publication No. Heisei 4-181651, etc. As descried in the above, relatively weak amorphous regions are ruptured through cold stretching thereby forming pores in a dry process in which these separating membranes of crystalline polymer are prepared, and a separating membrane prepared by this process has problems in that porosity is not sufficiently improved since porosity and pore size of the membrane are somewhat decreased. It is also difficult to uniformly control pore size and appearance of the membrane, and there is a limit in increasing the stretching ratio for maintaining a form of the separating membrane, even though only pure polymer is used in the manufacturing process because of the merits of a clean process in which problems such as solvent contamination, etc. do not exist at all. Furthermore, mechanical properties such as toughness are somewhat low due to anisotropy of stretching in the dry process.
Furthermore, a general polypropylene separator is prepared using stretching without solvents in a dry process, wherein a high permeability and desired mechanical properties together cannot be expected due to characteristics of polypropylene itself and the preparation method, since permeability and desired mechanical properties of the membrane generally tend to show inverse proportionality.
Therefore, it is an object of the present invention, considering problems of the conventional technologies, to provide a microporous membrane of high crystalline polypropylene having uniform pore size distribution, high pore density, and porosity.
It is other object of the present invention to provide a method for preparing a microporous membrane having uniform pore size distribution, high pore density, and porosity comprising the steps of preparing a precursor film using high crystalline polypropylene, annealing, low temperature stretching, high temperature stretching, and heat setting.
It is another object of the present invention to provide a separator for batteries, particularly a separator for lithium ion, or more particularly a separator for lithium ion polymer batteries having superior permeability characteristics and mechanical properties using the prepared microporous membrane.
It is another object of the present invention to provide a microporous membrane having a matrix of polypropylene prepared by stretching without using solvents, wherein both permeability and mechanical properties of the membrane are improved, and a method for preparing the same.
In order to accomplish the objects, the present invention provides a high crystalline polypropylene microporous membrane satisfying one or more physical properties selected from the group consisting of a crystallinity of 50% or more, isotacticity (pentad fraction) of 95% or more, an atatic fraction of less than 5%, a density of 0.905 g/cm3 or more, a melting temperature of 164xc2x0 C. or more, and a crystallization temperature of 125xc2x0 C. or more.
Furthermore, the present invention provides a preparation method of a microporous membrane comprising the steps of:
a) preparing a high crystalline polypropylene precursor film
satisfying one or more physical properties selected from the group
consisting of crystallinity of 50% or more, isotacticity (pentad fraction)
of 95% or more, less than 5% of an atatic fraction, a density of 0.905
g/cm3 or more, a melting temperature of 164xc2x0 C. or more, and a crystallization temperature of 125xc2x0 C. or more;
b) annealing the precursor film;
c) stretching the annealed precursor film at a low temperature;
d) stretching the low temperature stretched film at a high temperature; and
e) heat setting the high temperature stretched film.
Furthermore, the present invention provides a high crystalline polypropylene lithium ion battery separator satisfying one or more physical properties selected from the group consisting of crystallinity of 50% or more, isotacticity (pentad traction) of 95% or more, an atatic fraction of less than 5%, a density of 0.905 g/cm3 or more, a melting temperature of 164xc2x0 C. or more, and a crystallization temperature of 125xc2x0 C. or more.
Furthermore, the present invention provides a high crystalline polypropylene lithium ion polymer battery separator satisfying one or more physical properties selected from the group consisting of crystallinity of 50% or more, isotacticity (pentad fraction) of 95% or more, an atatic fraction of less than 5%, a density of 0.905 g/cm3 or more, a melting temperature of 164xc2x0 C. or more, and a crystallization temperature of 125xc2x0 C. or more.
Furthermore, the present invention provides a multi-component microporous membrane having a matrix of polypropylene prepared by stretching without using a solvent, wherein a synthetic resin component of the multicomponent microporous membrane comprises a) 70 to 99 weight % of polypropylene; and b) 1 to 30 weight % of one or more polymers selected from the group consisting of poly(ethylene-butylene) copolymer, poly(ethylene-hexene) copolymer, poly(ethylene-octene) copolymer, polyethylene prepared by metallocene catalysts, poly(ethylene-vinylacetate) copolymer, poly(styrene-ethylene) copolymer, poly(styrene-butylene-styrene) copolymer, poly(styrene-ethylene-butylene-styrene) copolymer, silane group grafted polyolefin, maleic anhydride or acrylic acid grafted polyolefin, ionomer, and derivatives thereof.
Furthermore, the present invention provides a preparation method of a multicomponent microporous membrane having a matrix of polypropylene prepared by stretching without using a solvent comprising the steps of:
a) mixing synthetic resin raw materials in a ratio of
i) 70 to 99 weight % of polypropylene; and
ii) 1 to 30 weight % of one or more polymers selected from the group consisting of poly(ethylenebutylene) copolymer, poly(ethylene-hexene) copolymer, poly(ethylene-octene) copolymer, polyethylene prepared by metallocene catalysts, poly(ethylene-vinylacetate) copolymer, poly(styrene-ethylene) copolymer, poly(styrene-butylene-styrene) copolymer, poly(styrene-ethylene-butylene-styrene) copolymer, silane group grafted polyolefin, maleic anhydride or acrylic acid grafted polyolefin, ionomer, and derivatives thereof;
b) preparing a precursor film by feeding the mixed material into an extruder;
c) annealing the precursor film at a temperature of polypropylene melting point or less;
d) low temperature stretching the annealed film at a temperature of ordinary temperature or less;
e) high temperature stretching the low temperature stretched film at a temperature of polypropylene melting point or less; and
f) heat setting the high temperature stretched film under tension at a temperature of polypropylene melting point or less.
Furthermore, the present invention provides a battery separator comprising the multi-component microporous membrane.
Furthermore, the present invention provides a lithium ion battery or particularly lithium ion polymer battery using the multi-component microporous membrane as a separator.